The use of gas bubbles for mixing the contents of a sample container is well known. U.S. Pat. Nos. 3,549,994 and 3,588,053 both teach such use of bubbles for the mixing of biological samples. The sample contains microscopic particles that are to be analyzed by passing the mixed sample through a microscopic path defined by an aperture in the wall of an analyzing vessel. U.S. Pat. Nos. 3,567,321 and 4,014,611 disclose forms of such analyzing vessel. Method and apparatus for accomplishing such analysis are taught in U.S. Pat. Nos. 2,656,508 and 3,259,842. All the above patents are directed to methods and apparatuses intended for use in a Coulter particle analyzing device. The mark "Coulter" is the Registered trademark, registration No. 995,825 of Coulter Electronics, Inc. of Hialeah, Florida. To the extent that a better understanding of the present invention may require, these six patents are incorporated herein as a part hereof by specific reference.
U.S. Pat. Nos. 3,549,994 and 3,588,053, especially the latter, teach that the mixing bubbles should be large and not break apart to form microscopic bubbles which might appear to the analyzing elements as being the microscopic particles which are to be analyzed. Also, the bubble induced mixing action should not be turbulent, which also would generate microscopic bubbles. According to the teachings of these two patents and commercial structures sold for many years, a continuous stream of gas is fed for the duration of the mixing by valves and control elements, including a needle valve, into the sample container. The continuous gas stream, when it enters the bottom of the container having therein liquid sample, breaks up into parts that then form relatively large bubbles. U.S. Pat. No. 3,588,053 states that the bubbles are on the order of 1,000 to 3,000 microns in diameter. The needle valve regulates the amount of entering gas and thus can control the amount of mixing action -- more gas resulting in more bubbles per unit time, within certain limits. Such bubble formation control has been satisfactory, but has required adjustment by the instrument operator to maintain sufficient but not turbulent mixing action. Heretofore, mixing by bubbles in a particle analyzer of the Coulter type was directed to blood particles, the smallest of which was the red blood cell having a typical volume of ninety cubic microns, which has an equivalent diameter of five and one-half microns. Hence, if the mixing bubbles and mixing action resulted in the generation of very small bubbles, smaller than sixty-five cubic microns, which has an equivalent diameter of five microns, those bubbles would not be mistaken for a red blood cell and in fact could be excluded by electronic threshold circuits.
However, the need to analyze small particles, such as blood platelets, which are much smaller than red blood cells, has brought with it the advent of more sophisticated particle analyzing equipment, having the capability of analyzing smaller particles than before, but also increased sensitivity to the generation of undesirable very small bubbles by the mixing arrangement above described. Although the quantity of these very small bubbles could be reduced by closing down the needle valve to produce fewer of the large mixing bubbles, there resulted insufficient mixing action. Attempts to reduce the number of mixing bubbles and to increase their size to obtain adequate mixing action by needle valving proved unsuccessful.